In the following, description is made by referring to an example of a flat plate microlens. A flat plate microlens is a lens array having a large number of fine lens portions having a higher refractive index than the surrounding portion formed by arrangement planarly within a glass substrate. It is generally prepared by coating the surface of a flat glass substrate with a masking material provided with an opening in a predetermined lens arrangement pattern, and permitting ions such as thallium (Tl) ions, cesium (Cs) ions, etc. to be internally diffused through the masking material opening by way of exchange with the ions within the glass substrate, thereby forming a lens portion with approximately semicircular cross-section having a refractive index distribution based on the concentration distribution of the diffused ions.
When an optical element is integrally formed within a glass substrate by ion diffusion as described above, local stress is generated on the diffusion surface side of the substrate due to the difference in ion radius between the diffused ions and the exchanged ions, whereby there is the problem that warping deformation occurs due to the difference in stress strain between both surfaces of the substrate. Particularly, when the thickness of the substrate is made thin as 1 mm or less, the amount of deformation becomes markedly large, whereby there is the problem that the lens substrate thickness cannot be made so thin. On the other hand, from an intention to make the device having a lens array assembled therein compact, it has been strongly demanded to make also the above-mentioned thickness of the substrate as thin as possible.
As one method to remove the stress strain difference between both surfaces of the lens substrate, there is also proposed a method which forms lens arrays with the same pattern on both surfaces of the substrate. However, according to this method, individual optical system is a combined lens comprising two lenses, whereby there is involved an essential problem that no desired optical performance can be obtained.
Having described above by referring to an example of lens, the situation is the same also when forming other optical elements such as light waveguide, etc. within a glass substrate.